We've lived in our house for over 30 years, and I haven't had this problem before when switching from heat to AC. Our house has underground cement air returns. We have a heat pump, so the same unit supplies both heating and cooling, as well as a plain fan option.
Until a few nights ago, the heat was coming on at night, and there was no problem. Then we had a heat spell and the house was so warm last night that I turned on the AC. The air blowing out the supply vents smelled musty and dusty. (Same ducts, unit, and filter that had been supplying heat with no problem.) This didn't go away after 15 minutes, so I turned everything off.
This morning I changed the filter, and ran just the fan. No AC, just the fan. I thought if the returns were damp or dusty, this would help. Also, I thought that this might identify a problem with the cooling part of the system - if there wasn't a problem running just the fan, then I would have to call someone to look at the cooling part of the system. However, I got the same musty, dusty odor when just the fan is running. After 20 minutes the odor was the same, so I turned on the heat, and sure enough, I could NOT smell the mustiness while the heat was blowing.
By the way, when I pulled the old filter out, it was NOT damp, and I just checked the new one that I ran with the fan -- totally dry.
Any ideas?
Until a few nights ago, the heat was coming on at night, and there was no problem. Then we had a heat spell and the house was so warm last night that I turned on the AC. The air blowing out the supply vents smelled musty and dusty. (Same ducts, unit, and filter that had been supplying heat with no problem.) This didn't go away after 15 minutes, so I turned everything off.
This morning I changed the filter, and ran just the fan. No AC, just the fan. I thought if the returns were damp or dusty, this would help. Also, I thought that this might identify a problem with the cooling part of the system - if there wasn't a problem running just the fan, then I would have to call someone to look at the cooling part of the system. However, I got the same musty, dusty odor when just the fan is running. After 20 minutes the odor was the same, so I turned on the heat, and sure enough, I could NOT smell the mustiness while the heat was blowing.
By the way, when I pulled the old filter out, it was NOT damp, and I just checked the new one that I ran with the fan -- totally dry.
Any ideas?
source https://hvac-talk.com/vbb/threads/2227245-Underground-air-return-AC-startup-question?goto=newpost
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